这是吴镇宇首次执导的影片，导演企图通过片中人物的交流、命运来揭示环境变迁、社会进步给人带来的枷锁，呼吁人们相互之间彼此信任。烂头（吴镇宇 饰）是一名普通的香港警察，他的佩枪编号9413,似乎冥冥中就注定了他的悲剧命运。烂头现在的精神几近崩溃，已经丧失了全部记忆。他唯一的办法就去寻求心理医生的帮助，他一直想逃离自己现在的生活状态——肮脏、迷乱的世界，但是他又似乎永远都无法回到自己过去那个干净明亮的世界……

When a human head is discovered, the paths of a female student, a cellist, an autistic teenager, a police officer, a prostitute, a teacher and a dog are found to be serendipitously entwined. Who caused the death? What is the motive? Where is the body? Attempts to answer these questions lead to keywords starting with the letter "G". As the case is peeled back layer by layer, the complexity and desperation of the people living in Hong Kong today are slowly uncovered, and a suspenseful multifarious Hong Kong story is revealed.